1. Field
The present invention relates to non-steady ph oto-induced electromotive force (EMF) sensors and, more specifically, to an apparatus and method for improving the performance of photo-EMF sensors by minimizing or eliminating spurious noise sources.
2. Description of the Related Art
A non-steady-state photo-induced electromotive force (photo-EMF) device can generate time-varying photocurrents in response to a corresponding lateral and rapid shift of an optical pattern across its surface. The optical pattern may have, for example, the form of a moving optical fringe pattern or a moving speckle pattern. Conventional photodetectors, on the other hand, respond only to changes in the incident light level, and not to the lateral motion of an optical pattern. For example, if there are several fringes incident on the surface of a conventional photodetector, there may not be any response to a moving pattern, since as each bright fringe leaves the active region of the conventional photodetector, another bright fringe may enter the active region at the same time from the opposite side of the active region.
A photo-EMF sensor resembles a conventional semiconductor detector; however, to provide the pattern response characteristics described above, it can develop an internal lateral electric field that stores the spatial intensity pattern of an incident optical beam. FIG. 1 shows a conventional photo-EMF sensor 100 comprising a detection substrate 110 and a pair of surface electrodes 120. To provide the photo-EMF function, the detection substrate 110 is typically formed of a semi-insulating photoconductor with sufficient carrier trap density to form an effective space charge grating. FIG. 1 also shows an intensity pattern 150 of an incident laser beam 152 with the resulting lateral internal space charge field 160 developed in the material between the electrodes 120. The detection substrate 110 may comprise gallium arsenide and the surface electrodes 120 may comprise a titanium/gold alloy.
One application of a photo-EMF sensor is in a velocity, or differential, sensing interferometer system, in which the photo-EMF sensor acts as a fringe processing unit. See, for example, the system described by Pepper et al. in U.S. Pat. No. 5,909,279, “Ultrasonic Sensor Using Short Coherence Length Optical Source, and Operating Method,” issued Jun. 1, 1999. In such a system, interference fringes are directed onto the surface of a photo-EMF sensor. Photo-generated carriers are then produced which diffuse away from regions of intense optical radiation. These carriers become trapped and formed a periodic charge pattern and the corresponding space-charged field. In the absence of any change in the interference pattern, the space charge field is static and no net current is produced from the photo-EMF detector. However, if the interference pattern changes at a sufficiently high rate, the space charge field can not track the changes, and a net current is produced by the photo-EMF sensor.
Hence, a key feature of a photo-EMF sensor is that it senses rapid motion of an optical pattern across its surface by generating a photo-EMF current in response to the rapidly moving pattern. As noted above, the pattern may be a set of interference fringes resulting from the interference of two light beams, and the rapid motion of the pattern may be the result of one of the beams experiencing a dynamic phase shift. In a laser-ultrasound probing system, the dynamic phase shift may be the result of motion in a probed surface. In a laser communication receiver, the motion may be due to phase-modulation encoded onto a light beam. The photo-EMF sensor is adaptive, since static and slowly varying changes, such as those due to beam wander, vibrations, thermal effects, turbulence, are adaptively tracked.
The photo-EMF sensor will produce no current for such changes, as long as the space-charge field formation time is faster than the changes. Thus, the photo-EMF sensor can adaptively compensate for the effects due to these changes. This adaptive compensation capability makes the photo-EMF sensor extremely attractive for multiple applications, since the sensor detects high-bandwidth information, while suppressing low bandwidth noise.
Photo-EMF sensors known in the art may comprise a plurality of interlaced electrode pairs, e.g., interdigitated contacts. See, for example, U.S. Pat. No. 6,342,721, issued Jan. 29, 2002 to Nolte et al. See also Nolte et al., “Enhanced Responsivity of Non-Steady-State Photoinduced Electromotive Force Sensors Using Asymmetric Interdigitated Contacts,” Optics Letters, vol. 24, no. 5, March 1999, pp. 342-344. These references disclose the use of multiple contacts with alternating wide and narrow active-area spacings, with the currents from the wide-spaced active area regions summed, while the narrow areas are optically blocked or rendered insensitive. The use of multiple contacts helps improve the response of the photo-EMF sensor, while using narrow-active areas that are optically blocked or rendered insensitive helps decrease the back action current that may arise from proximate electrodes.
However, in photo-EMF sensors known in the art, even those using multiple electrodes, the photo-EMF sensor may not always track out (or, adapt to) all the undesirable noise, owing to photovoltaic (i.e., band bending) effects at or near the electrodes. Those skilled in the art recognize that a band-bending region exists as a small region near a metal contact on semiconducting material, which has an electric field that emerges from the contact and goes into an active area. This band-bending region arises because the Fermi energy of the metal pins at a surface energy different than a Fermi level of the semiconducting material. The electric field may exist over a limited distance from the contact, which may be in the range of several microns from the contact. The band-bending region and the photovoltaic effect are well known to those skilled in the art of conventional semiconductors and photodetectors.
However, in photo-EMF sensors known in the art, the response of the photo-EMF sensor to an optical pattern that travels through the band-bending region is different than the response of the sensor as the optical pattern travels through other portions of the active region or regions of the sensor. That is, as the optical pattern moves through the electric field gradient due to the photovoltaic effect, a measurable current is generated, even at low frequencies of the lateral motion of the pattern. This effect may result in large-voltage output level variations from the presence of dynamic and spatial amplitude changes in the optical intensity near the electrodes, whose time scales can range from DC (a fixed pattern feature near an electrode) to the upper limit of the detection bandwidth. As such, these noise sources are not adaptively compensated. This can result in significant saturation of subsequent high-gain amplifiers, as well as a degradation in the system dynamic range. Simply AC coupling the sensor to the amplifiers will not eliminate this problem, especially in the case of signals whose bandwidths are similar to the photovoltaic induced noise.